The use of composite materials in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength-to-weight ratio and high stiffness, making them attractive for use in lightweight aircraft structures. Some drawbacks to using composite materials have been high fabrication costs and low damage tolerance. Generally, it has been difficult to produce parts formed of high-strength composite materials at the same cost as comparable metal parts. It has also generally not been possible to produce composite parts having the same degree of damage tolerance as comparable metal parts. These cost and damage tolerance differences between composite and metal parts are especially notable in large scale parts having complex contours.
Another disadvantage of composite materials has been their relatively low temperature tolerance. The introduction of thermoplastic composite materials has increased the temperature tolerance of composites. In addition to having higher temperature tolerance, thermoplastic composites are impervious to most chemicals and have superior strength in some applications making them ideal candidate materials for advanced aircraft. However, manufacturing difficulties have generally made it difficult, if not impossible, to fabricate complex parts from such thermoplastic composite materials.
One of the contributors to such fabrication difficulty has been the form in which most thermoplastic composite materials are obtained from a material supplier. Generally, thermoplastic composite materials come in rolls of flat material having unidirectional reinforcement fibers embedded within a stiff, polymerized plastic matrix. Such rolls are currently available in widths ranging from approximately a 1/4 inch to a foot. The thermoplastic matrix material is in a solid state, thus resulting in a slick, flat, nonformable material. The stiffness of the thermoplastic composite material prevents it from being easily bent or formed around complex contours. In addition, even if formed around a contour, the thermoplastic material maintains a memory such that once the forming pressure is released the material returns to its flat shape.
The stiff, nondeformable nature of most prior thermoplastic composite materials have relegated them to use in simple parts having gentle contours such as flat panels, fuselage doors, etc. It would be advantageous if thermoplastic composite materials could be used in highly stressed primary aircraft structure such as ribs and spars. In the past, the complex contours of most aircraft's primary structure has prevented thermoplastic composite materials from being used. Due to the stiff, nondeformable nature of the thermoplastic composite materials, it has generally not been possible or cost-effective to produce high-quality parts having such complex contours. One of the primary aircraft structures that could be advantageously formed from thermoplastic composite materials is sine wave spars for aircraft wing structures.
Aircraft spars can be formed of C-channels, I-beams, or I-beams having sine wave central webs. Sine wave spar configurations have been found to provide superior weight and strength properties in most aircraft structures when compared to other spar shapes. In order to form sine wave spars of thermoplastic composite materials, the thermoplastic composite materials must be formed around complex sine wave contours and sharp radius corners in order to form the web and caps of the spar. Prior art manufacturing techniques have not been able to produce high-quality formed sine wave spars from thermoplastic composite materials.
One method tried to overcome fabrication difficulties with thermoplastic materials is the use of a cloth composite material having a thermoplastic matrix material that is applied to the reinforcing fibers in a powder form. During processing, the thermoplastic matrix material is heated to a temperature at which the matrix material flows together to form a consolidated thermoplastic composite part. However, such cloth materials are expensive, and have been prone to problems associated with fiber wet-out, voids, etc. In addition, the use of cloth materials is not as structurally efficient as the use of unidirectional composite materials.
As can be seen from the discussion above, there exists a need for methods and apparatus to form thermoplastic composite materials into complex shapes such as sine wave spars for use in aircraft applications. The present invention is directed toward meeting this need.